Biological equivalent dose prediction in pion irradiation using pion star density and the mixed beam lesion additivity model.

The biological equivalent dose profile of the pion beam was predicted with one parameter (gamma) using physical dose and pion star density. The value of the relative biological effectiveness (RBE) at each depth was given as a linear function of pion star density (PSD): RBE = 1.0 + gamma PSD, assuming (i) the mixed beam lesion additivity model and (ii) the linear relationship between the ratio of high LET dose to total dose and pion star density. The predicted depth-survival curve fitted well with the pooled biological data of the gel technique using Chinese hamster cells (CHO). The predicted RBE were consistent with previously published results through the flat dose peak, except at the decreasing portion of the dose profile. The practical usefulness of this model in clinical treatment is stressed.